The present invention relates to seamless steel tubes to be used as hollow shaft blanks which are better fitted to reduce the weight of drive shafts used in automobiles, and more particularly to seamless steel tubes having excellent, cold workability, hardenability, toughness and torsion fatigue strengths as well as being most suitable as starting materials for making hollow drive shafts by applying heat treatment subsequent to cold swaging of both ends thereof, and a method for producing the same.
From the view point of global environment protection, it is highly demanded to reduce the weight of car-body to improve the fuel efficiency. In this regard, there have been various trials that solid members among automobile parts are replaced with hollow members. In these trials, a drive shaft which transmits the driving force to the wheel is also attempted to be made from a hollow blank.
The purpose of making automobile parts to have a hollow structure is not only to reduce the weight thereof but also to expectedly improve an acceleration response owing to the enhancement of torsion stiffness and to expectedly control an indoor quietness in a moving car owing to the improvement of vibration characteristics as well, which is expected to be fulfilled at any rate, and a strong demand for developing hollow shafts processed in a special shape is growing in association with the fulfillment thereof.
For instance, in a design that both shaft ends are securely fixed to constant-velocity joints, an intermediate portion of the shaft is thin in wall thickness and has a large diameter as much as possible, whereby not only the torsion stiffness is enhanced but also the vibration characteristics are improved. In the meantime, by setting the diameter of both shaft ends—to be securely fixed to constant-velocity joints—to be equal to the diameter of solid members which have been used to date, existing constant-velocity joints can be utilized as they are.
As a manufacturing method of hollow drive shafts, there is a method that a hollow or solid shaft is securely fixed to both ends of a hollow tube blank by means of friction welding or the like. However, this method cannot be applied for the case that the hollow portion has a large diameter but the diameter at both ends is small. By reason mentioned above, in order that a drive shaft may be formed in such a manner that an intermediate portion thereof is configured to have a thinner wall thickness and larger diameter as much as possible and the diameter at both ends is small, it is attempted to make one-piece type hollow drive shafts by applying following procedure: steel tube blanks are subjected to cold working for wall thinning in the intermediate portion thereof; and subsequently, both ends of steel tube blanks are subjected to cold reducing etc. to not only reduce the tube end outside diameter but also increase the wall thickness at both ends.
Meanwhile, the one-piece type hollow drive shaft mentioned above is subjected to complex cold working so as to be formed into the specialized unique shape. Accordingly, when welded tubes are used as steel tube blanks to make hollow drive shafts, there is an issue that any cracking should occur along the weld line during forming operation and/or any fatigue crack develops along the weld line in the fatigue test to be conducted after forming operation. Thus, at present there is insufficient reliability in using welded tubes as hollow shaft blanks for making hollow drive shafts.
Therefore, to prevent any cracking during a forming operation by means of cold working and to secure sufficient torsion fatigue strengths after a forming operation, there is a growing demand for using seamless steel tubes as hollow blanks for making one-piece type hollow drive shafts. To respond to the demand, there are proposed hollow drive shafts adopting seamless steel tubes as hollow shaft blanks.
When one-piece type hollow drive shafts are made by using seamless steel tubes as hollow shaft blanks, it is important to prevent any cracking attributable to a reducing process and/or spinning process for tube ends. Furthermore, it is required to harden through the whole thickness from the outside surface to the inside surface and secure high toughness by means of heat treatment subsequent to cold working, and also required to secure sufficient torsion fatigue strengths to allow a longer service life for the final product.
In other words, when seamless steel tubes are used as hollow shaft blanks for making hollow drive shafts, it becomes indispensable that excellent cold workability which allows to form complex shapes, excellent hardenability and sufficient toughness in association with heat treatment, and sufficient torsion fatigue strength are concurrently satisfied. However, in hollow drive shafts which have been proposed thus far, the metallurgical aspect of seamless steel tubes has been hardly focused and studied.
For instance, Japanese Patent Application Publication No. 06-341422 discloses drive shafts in which a balance weight is fixed to a steel tube used in a drive shaft so as to reduce a revolution-related run out amplitude, wherein Carbon Equivalent (Ceq=C+Si/24+Mn/6+Cr/5+Mo/4+Ni/40+V/14) is set forth for the steel tube for the drive shaft and for the balance weight as well, so that any fatigue failure developing from the portion to which the balance weight is welded can be suppressed.
Nonetheless, it is not possible to obtain seamless steel tubes having excellent cold workability as well as excellent fatigue characteristics by simply stipulating Carbon Equivalent (Ceq) for the steel tube for the drive shaft and for the balance weight. By reason of this, it is difficult for the automobile propeller shaft disclosed in the Japanese Patent Application Publication No. 06-341422 to be applied as an one-piece type hollow drive shaft.
Next, in the Japanese Patent Application Publication No. 07-018330, there is disclosed a method for manufacturing high strength and toughness steel tubes suitable for the high strength member used in skirt members of automobiles. In the disclosed method, detail chemical compositions are stipulated while Ti is not contained and N is not specified at all, whereby even if B may be added, the steel composition is not configured to sufficiently impart harden ability. Further, the steel compositional design is not made in consideration of cold workability and fatigue characteristics, so that the manufacturing method disclosed in the Japanese Patent Application Publication No. 07-018330 is unlikely applied to produce seamless steel tubes as starting materials suitable for one-piece type hollow drive shafts.
Further, in the Japanese Patent Application Publication No. 07-088537, there is disclosed a method for manufacturing one-piece type hollow drive shifts wherein steel tubes with irregular inside diameters are made from tube blanks by cold drawing for wall thinning in which the plug outside diameter and die inside diameter are stipulated. However, the material grade disclosed in EXAMPLES is carbon steel corresponding to S48C specified in JIS Standard, and it seems that there is no intention to stipulate specific chemical compositions for purpose of improving cold workability, hardenability and fatigue characteristics.
And further, in the Japanese Patent Application Publication No. 08-073938, there is disclosed a method for producing high strength and toughness steel tubes, comprising the steps of: applying cold working by 10-70% in cross-section area reduction rate after hot tube making process; annealing; and heat-treating in combination of induction hardening and subsequent tempering. In the manufacturing method disclosed by the Japanese Patent Application Publication No. 08-073938, detail chemical compositions of steel stocks to be used are stipulated, but similarly to the manufacturing method described in the Japanese Patent Application Publication No. 07-018330, even if B and/or Ti may be added, the steel composition is not configured too sufficiently impart hardenability and further, the steel compositional design is not made in consideration of cold workability and fatigue characteristics, so that it is unlikely applied to produce tube blanks suitable for one-piece type hollow drive shafts.
Meanwhile, in the Japanese Patent Application Publication No. 2000-204432, there are disclosed drive shafts wherein induction hardening is applied to graphite steel so as not only to harden the surface layer but also to form a dual phase structure composed of ferrite and martensite in the core area. However, chemical composition disclosed in the Japanese Patent Application Publication No. 2000-204432 is suitable for hollow drive shafts made by means of friction welding and the heat treatment accompanying longer duration is required in order to obtain graphitized steel. In addition, since Cr is not contained in the chemical compositions, hardenability as well as fatigue strengths are not sufficient, whereby this is not; pertinent to steel tubes suitable for one-piece type hollow drive shafts.
And the Japanese Patent Application Publication No. 2001-355047 teaches high carbon steel tubes having excellent cold workability and induction hardenability as tube blanks for drive shafts, wherein the grain size of cementite is controlled to be not more than 1 μm. However, in this high carbon steel tubes by the Japanese Patent Application Publication No. 2001-355047, warm working is required to obtain the targeted microstructure to thereby increase production costs, and what is more, the disclosed chemical compositions are not pertinent to one-piece type hollow drive shafts which should concurrently satisfy cold workability, hardenability and fatigue characteristic.